Electrochemical cells able to convert chemical to electrical energy are a well known source of energy. Electrochemical cells which are based on alkali metal ions, more specifically on lithium ions, have been found to have very advantageous characteristics, such as high energy density per unit volume or unit weight. Lithium electrochemical cells can operate as primary cells, but more often utilization is in the form of secondary, rechargeable cells or rechargeable batteries. Lithium batteries or cells may be button shaped, cylindrically or prismatically wound, or flat, composed of layers, known as planar or laminar cells or planar batteries. Lithium electrochemical cells or batteries include a negative electrode or anode, containing a substance capable of intercalating lithium, or lithium metal or an alloy of lithium, as the negative active component. The positive active component of the cathode is usually a chalcogenide of a transition metal and lithium, such as lithium-manganese oxide, lithium-cobalt oxide, or similar compounds which are stable under the conditions of operation and can incorporate lithium ions in their structure reversibly. A lithium cell and/or battery commonly has a non-aqueous electrolyte which may be a porous separator impregnated with an organic liquid or gel containing a dissolved lithium salt, or a solid polymer laminate containing a dissociable lithium compound, or granules of electrolyte mixed with particles containing one of the electrode active compounds. Lithium batteries are usually equipped with current collectors in close proximity of the electrodes, which can be a metal plate, rod, metal foil carried by a polymer laminate, or a similar electrically conducting structural element.
The energy output of a cell or battery is strongly affected by the manner of collecting the current generated by the electrochemical reaction. There are known methods to improve contact between the metallic current collectors and the adjacent electrodes. Methods of etching, cleaning or micro-roughening the surface of the current collector in contact with the electrode have been described. Inserting a separate layer of an electronically conducting polymer or a polymer loaded with electrically conducting carbonaceous or inorganic particles, between the metallic current collector and the electrode of the electrochemical cell are known. Methods are also known to increase adhesion between the electrode and the current collector. The adhesion promoting layer can be coated onto the current collector as a polymer coating carrying conductive particles, which may be subsequently cured by known methods or the solvent in the polymer solution is evaporated. The electrically conducting particles in the adhesion promoting coating are most frequently fine carbon or carbon black, but may be fine particles of metal or electrically conducting oxides, or such like. Adding conductive binders or admixing carbon or metallic particles or metal coated carbon fibres to increase the conductivity of the electrode mixture itself are known. Examples of lithium batteries incorporating some of the above discussed features are provided in U.S. Pat. No. 5,464,707, issued to Moulton et al. on Nov. 7, 1995, in U.S. Pat. No. 5,589,297, issued to Koga et al. on Dec. 31, 1996, and in U.S. Pat. No. 5,554,459, issued to Gozdz et al. on Sep. 10, 1996. U.S. Pat. No. 5,824,120, issued to Mitchell et al. on Oct. 20, 1998, describes an electrically conductive adhesion promoter layer located between the current collector surface and the cathode. The adhesion promoter layer of Mitchell et al. is made of a polymeric material, such as acrylics or acetates, containing conductive particles which may be graphite particles, hollow carbon fibres, ultrafine metal powders, metal coated carbon fibres, metal coated or conductive polymeric fibres or inorganic particles, such as indium tin oxide.
Carbon or graphitic particles capable of intercalating lithium ions or carbon fibres are frequently utilized in the anode mixed into a paste with an tonically conducting binder. Ohsaki et al. in U.S. Pat. No. 5,856,043 teach a lithium cell anode made of 87-95% vapour-grown graphitized carbon fibres mixed with a binder. The conductivity of the cathode is adjusted by the addition of carbon black.
As discussed above, fine carbon particles have been deployed as a means of increasing electronic conductivity between the current collector and the positive active material, either in a coating or layer directly in contact with the current collector, or mixed with the oxidic or sulphidic electrode-active material. The individual particles of carbon, however, may be isolated from one another or from the current collector by other components in the mixture forming the electrode coating or the electrode paste, thus reducing the effectiveness of electron transfer. Moreover, the resistance of current collector surface in touch with the electrode or the conductivity enhancing layer may have local variations, hence increase in lateral conductivity can be a desirable feature.